SurfaceFlinger: frame rate heuristic during config change

Some devices might takes a few frames to change the refresh rate.
Calculating the frame rate should ignore frames sent while changing
the refresh rate as those might result in an inaccurate frame rate
due to frame misses.

Bug: 156530990
Test: Running Hay Day and observing refresh rate.
Change-Id: I99f364cd1816c3ce2ba3c3145331eb618dcaea71